Bytes of data often describe graphics images as an arrangement of raster rows. A raster row is one line or strip of a graphics image. Numerous parallel raster rows may be arranged or stacked to produce an entire graphics image.
Each byte of data describes a part of a raster row within the graphics image. Because each byte describes a part of a raster row, the data itself may be referred to as a raster row of data, or data arranged on a raster-row-by-raster-row basis, as is done in the following text.
Because graphics images are often complex, the data describing the images may be extensive and detailed. Accordingly, transferring such data between components in a system, such as between a host computer and a printer, often requires a large amount of memory and may result in an undesirable time lag. To limit the required memory and to prevent such time lags, the data may be compressed so that information is transferred by employing less data, with time thereby saved.
Data may be compressed in a variety of ways. For example, Gilbert Held, in his book Data Compression, published by Wiley Hayden, copyright 1984, discloses a compression method wherein bits of data within one raster row are compared with bits of data in an adjacent raster row. By comparing the bits, the differences between the rows may be noted. Once noted, one raster row may be described by stating how the bits in that row differ from the bits in the other row. The data describing the differences may be less extensive than the data describing the raster row, resulting in a compression of data.
However, comparing different raster rows of data on such a bit-by-bit basis does not recognize repetitious data within the raster rows themselves. Additionally, such a comparison does not lend itself to a concise method of recording the bit changes between the rows.
The method of the present invention overcomes the limitations of such prior data compression methods by recognizing repetitions within raster rows as well as by providing a concise method of recording changes between rows.